a) Field of the Invention
This invention relates to a purified chitin which, when dissolved in water, provides an aqueous solution of the chitin, said solution having low turbidity and high transparency; and also a production process of the purified chitin. The aqueous solution can be an aqueous acidic solution. Throughout this specification, chitin, chitosan, chemically modified products (derivatives) thereof and degradation products thereof may be collectively called “a chitin”. However, the expression “aqueous solution of a chitin” does not include “an aqueous solution of chitin” as chitin is soluble in neither water nor diluted acid.
b) Description of the Related Art
Chitin is a natural high molecular substance, which exists in the exoskeletons of crustaceans, the calcified internal shells of cuttlefishes, fungi and molds. Industrial production of chitin is actively performed these days. A large majority of its sources are the shells of crabs, lobsters and shrimps, and the shells of squillas and the calcified internal shells of cuttle fishes are also used for certain applications. In the sense of industrial sources, there is no source other than the those mentioned above.
Deacetylation of chitin obtained from such a source affords chitosan. Using both of them as starting materials, various derivatives thereof and their oligosaccharides and monosaccharides are produced. As mentioned above, it is the current circumstances that as industrial sources for chitins, the shells of crabs, lobsters and shrimps are primarily employed with limited use of the shells of squillas and the calcified internal shells of cuttle fishes. These marine organisms obviously contain fatty substances.
Main components of the exoskeletons of crabs, lobsters and shrimps are calcium carbonate, proteins and chitin, each of which also contains fatty substances as a trace component. No economical and appropriate solvent is, however, known for the dissolution of chitin upon isolation of chitin from these exoskeletons. As an industrial method for obtaining chitin from their exoskeletons, it is hence a current practice to make the calcium carbonate and proteins in the exoskeletons soluble in water and then to remove them from the exoskeletons to obtain chitin. Described specifically, the exoskeletons of crabs, lobsters or shrimps are immersed in a dilute aqueous alkaline solution and subsequent to heating, degraded proteins are washed off with water. Insoluble matter with chitin contained therein is immersed in a dilute aqueous solution of hydrochloric acid to convert calcium carbonate, which is still contained in the insoluble matter, into calcium chloride which is soluble in water. The insoluble matter is then washed with water to remove the calcium chloride, so that chitin is obtained as insoluble matter. In this process, the treatment with the dilute alkaline solution and the treatment with dilute hydrochloric acid may be reversed in sequence.
Chitin, which has been obtained as described above, can be deacetylated into chitosan by immersing it, either after drying or without drying, in a conc. aqueous solution of sodium hydroxide, said solution being of such a concentration as giving a concentration of 40 wt. % after the reaction, and then heating it there. When chitosan obtained as described above is dissolved in a dilute aqueous solution of acetic acid and the resulting solution is examined for turbidity, the turbidity substantially fluctuates depending on the kind, growing area and the like of its source. In this sense, industrially-available chitosan lacks uniformity in quality and therefore, involves a problem as an industrial product. Depending on the source, an aqueous solution of the resulting chitosan has substantial turbidity. Such chitosan cannot be used as a commercial product, and may have to be discarded in some instances.
Using the above-obtained chitin or chitosan as a starting material, low molecular weight chitin, low molecular weight chitosan, glucosamine, glucosamine hydrochloride, glucosamine sulfate, acetylglucosamine and the like can be obtained by subjecting it to chemical degradation such as acid hydrolysis or oxidative degradation or to enzymatic degradation with chitinase or chitosanase in water by conventionally-known methods.
Further, from reactions of chitin or chitosan with epichlorohydrin, ethylene oxide, propylene oxide, glycidol or the like, the corresponding hydroxyalkyl derivatives are available; from reactions with monochloroacetic acid, carboxymethyl derivatives are available; from reactions with succinic anhydride, carboxyacyl derivatives are available; from reactions with glycidyltrimethylammonium chloride, quaternary ammonium derivatives are available; and from reactions with carbon disulfide, xanthate derivatives are available, all by conventionally-known methods. From reactions of chitosan with organic acids such as acetic acid, lactic acid and pyrrolidonecarboxylic acid, the corresponding organic acid salts of chitosan are available. Furthermore, hydrolysis of the xanthate derivative of chitin or chitosan regenerates chitin or chitosan, and neutralization of an organic acid salt of chitosan regenerates chitosan.
Aqueous solutions of water-soluble derivatives or degradation products obtained from chitosan, which produces turbidity when formed into an aqueous solution as mentioned above, or chitin as its raw material are often turbid, and these turbid aqueous solutions of chitins are significantly reduced in commercial value and those having substantial turbidity can find no commercial value.
No report has been made to date on the essential cause of the above-described problem, to say nothing of its resolution method. It is the current circumstance that each manufacturer of a chitin is trying to deal with this problem of turbidity by carefully choosing for the chitin a source that is empirically considered to develop less turbidity. Keeping in step with the active use of chitins in recent years, especially with their increasing use in fine chemicals, there is a growing desire for the resolution of the problem of turbidity in aqueous solutions of the chitins.